LRUCache.h

/*
Copyright [2024] [Yao Yao]

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

*/

#pragma once
#include <unordered_map>
#include <list>
#include <memory>
#include <cassert>
#include <optional>

namespace cudapp {
template <typename Key, typename Value>
class LRUCache
{
public:
    LRUCache(size_t capacity) : mCapacity{capacity} {}
    std::optional<Value> get(const Key& key) {
        std::lock_guard<std::mutex> lk{mMutex};
        const auto iterMap = mMap.find(key);
        if (iterMap == mMap.end()) {
            return std::nullopt;
        }
        assert(iterMap->first == key);
        const auto iterEntry = iterMap->second;
        assert(iterEntry->first == key);
        mEntries.splice(mEntries.end(), mEntries, iterEntry);
        return iterEntry->second;
    }
    void put(const Key& key, Value value) {
        std::lock_guard<std::mutex> lk{mMutex};
        mEntries.emplace_back(key, std::move(value));
        mMap.try_emplace(key, std::prev(mEntries.end()));
        while (mEntries.size() > mCapacity) {
            const auto& key = mEntries.front().first;
            assert(mMap.at(key) == mEntries.begin());
            invalidateImpl(key);
        }
    }
    void invalidate(const Key& key) {
        std::lock_guard<std::mutex> lk{mMutex};
        invalidateImpl(key);
    }
    void clear() {
        std::lock_guard<std::mutex> lk{mMutex};
        mEntries.clear();
        mMap.clear();
    }
private:
    void invalidateImpl(const Key& key) {
        const typename std::list<Entry>::iterator iterEntry = mMap.at(key);
        assert(iterEntry->first == key);
        mMap.erase(key);
        mEntries.erase(iterEntry);
    }
private:
    mutable std::mutex mMutex;
    size_t mCapacity;
    using Entry = std::pair<Key, Value>;
    // old to new
    std::list<Entry> mEntries;
    std::unordered_map<Key, typename std::list<Entry>::iterator> mMap;
};
} // namespace cudapp